Electrically active films, which contain nanocrystalline, nanoporous transition metal oxides or transition metal chalcogenides or their lithium inclusion compounds, are used as electrodes in primary or secondary electrochemical generators, as described for example in patent application WO 99/59,218. These electrically active films are further used in photovoltaic cells or in electrochromic displays.
In patent application EP 0,709,906 a positive electrode is described consisting of sintered particles of oxygen containing lithium compounds of a size of 33 μm. These particles are heated under pressure at a temperature between 350° C. and 1000° C. in order to attain the required electrical activity.
In U.S. Pat. No. 5,604,057 a cathode is described consisting of amorphous, nanoporous particles of manganese oxide including lithium ions of sub micrometer size having an internal surface greater than 100 m2/g. These electrodes are prepared by coating the manganese oxide together with a preferentially conductive binder or a mixture of binders onto a conductive support (as for example an aluminium foil). Coating may be effected by spraying, spin coating, blade coating or painting. Subsequently the coated support has to be heated, but not to a temperature exceeding 400° C. in order to prevent crystallization of the manganese dioxide.
In U.S. Pat. Nos. 5,211,933 and 5,674,644 a preparation method for LiMn2O4 with spinel structure and for LiCoO2 with a layer structure is described, wherein the compounds are prepared from acetate precursors. The powder of LiMn2O4 prepared in this way consists of particles having a size from 0.3 μm to 1.0 μm. Pressed tablets of this powder, with the addition of 10% of small graphite particles, are used as positive electrode in lithium batteries. Conductivity between the individual tablets is caused by the graphite particles. LiMn2O4 and LiCoO2 may also be coated onto a support together with a binder. Afterwards the coated support is heated to a temperature of 600° C. During this heating step, the organic binder is completely removed from the layer. The finally obtained electrically active film therefore no longer contains any organic binder.
In U.S. Pat. No. 5,700,442 inclusion compounds of manganese dioxide are described which are used as positive electrode in lithium batteries. These inclusion compounds are prepared by heating of a β-MnO2 powder with a lithium compound at a temperature between 150° C. and 500° C. for a sufficiently long period of time. The particles prepared in this way are relatively coarse and have a specific surface below 7 m2/g and are therefore not suitable as a material for active electrodes that may be discharged rapidly.
In patent application EP 0,814,524 lithium manganese oxides with spinel structure are described as active cathode material in secondary lithium ion batteries. The mean size of the particles is between 1 μm and 5 μm and the specific surface is between 2 m2/g und 10 m2/g. Due to the coarse particles and the low internal surface, these compounds are not suitable as a material for active electrodes that may be discharged rapidly.
In patent application WO 99/59,218 the preparation of electrodes consisting of TiO2 and LiMn2O4 with the aid of a coating or printing process is described, wherein an aqueous suspension of a manganate precursor compound is deposited onto a substrate. This coating operation normally has to be repeated several times in order to obtain the required layer thickness. Subsequently the coated substrate has to be heated to a temperature between 400° C. und 750° C. in the presence of air for a few minutes in order to obtain the required conductivity. The sintering process at high temperatures leads to layers with higher electrical activity than a sintering process at lower temperatures.
The preparation of thin film electrodes at room temperature by coating aqueous suspensions of manganate powers containing 3% of polyvinyl alcohol and 10% of graphite onto conductive supports is mentioned in “Influence of the Particle Size of Electrode Materials on Intercalation Rate and Capacity of New Electrodes”, Journal of Power Sources 81-82, 621-626 (1999). The layers are heated subsequently to a temperature of at least 200° C. during 15 minutes.
The known methods for the preparation of transparent conductors are listed in “Criteria for Choosing Transparent Conductors”, MRS Bulletin 25, 52 (2000) (Table 1). Most of the preparation methods need high temperatures up to 1000° C.
All these preparation methods do not allow a cost effective production of electrically active films due to the required high temperatures and the long process times. In particular, it is not possible to use cheap, transparent, commercially available supports that are used for example in the photographic industry, because they would be destroyed at the high temperatures required. There is the further disadvantage of the required high temperatures that heat-sensitive compounds, in particular organic compounds such as spectral sensitizing dyes cannot be added to the electrically active films. Furthermore the mechanical stability of such electrically active films is not sufficient as well as the adhesion of the layers to the substrate.